a) Field of the Invention
The present invention is directed to an arrangement and a method for determining the two-dimensional distribution of fundus pigments, particularly of the macular pigment xanthophyll. Xanthophyll has a positive effect on prevention of age-related macular degeneration (AMD), since, on the one hand, it is an absorber of high-energy short-wave radiation and therefore leads to reduced formation of free radicals and, on the other hand, is itself an effective radical scavenger. Therefore, it is presumed that a reduced optical density of xanthophyll represents an increased risk factor for incidence of AMD.
b) Description of the Related Art
According to the known prior art, there is a range of subjective and objective methods for determining the optical density of xanthophyll pigment in the macula.
In the flicker matching method of Bone and Hammond [1], local areas inside and outside the fovea are alternately irradiated by light of a suitable wavelength. The irradiation intensities are varied until the patient subjectively perceives the identical brightness. The optical density of the xanthophyll in the irradiated area of the macula can then be determined from the ratio of the foveal and parafoveal irradiation intensities. This method has the drawback that it can only provide locally defined information about the optical density. However, its decisive disadvantage consists in that it is a subjective method, i.e., it relies on the cooperation of the test subject. The method assumes the subject's capability for fixation and cooperation and therefore can be realized only with difficulty for the typically older patients and can not be carried out at all with subjects having foveal fixation loss (e.g., with AMD).
The known objective methods of fundus spectroscopy according to Berendschot, van der Kraats and Schweitzer [2] are based on the evaluation of the reflectance spectra in a selected retinal area or on the evaluation of difference spectra at two spatially separate (foveal, parafoveal) retinal areas. This method has the disadvantage that only pointwise local measurements or measurements only along a line are possible. Further, this method involves expensive apparatus and is therefore unsuitable for extensive practical application.
In the so-called two-wavelength method according to Elsner [3], reflection images of the retina are recorded by laser scanners in two different wavelengths and the absorption maximum of xanthophyll is determined by forming the quotient while taking into account the relative absorption proportions. While this objective measurement method makes it possible to determine the two-dimensional distribution of the optical density, the measurements can be falsified by the transparency of the front ocular media which varies depending upon wavelength.
Delori [4] describes another objective method in which it is possible, in principle, to detect the two-dimensional distribution by measuring the attenuation of the fluorescence intensity of submacular endogenous fluorophores. However, because of the very low autofluorescence intensity of endogenic fluorophores, expenditure on apparatus is extremely high. Further, the measurements can likewise be falsified by the varying transparency characteristics of the front ocular media.